Ok, finally framed the #monkeyisland #puzzle.
Apparently the glue was not dry enough and the cardboard became slightly larger because of it.
A few hours later and now it fits inside the frame. Tight fit, but a fit nonetheless.
Now I just need to hang it. I really want to start the Monkey Island 2 one, but I promised to my cats that they can come back in the living room until Sunday. So I'll have to wait...